The present invention relates generally to electromagnetic interference (EMI) shielding in electronic devices and deals more specifically with EMI shielding for portable electronic devices such as cellular or radio telephones, and in particular with a molded cover having a releasable EMI shield for such electronic devices.
Portable electronic devices, particularly portable handheld communication devices such as cellular or radio telephones and the like require electromagnetic interference (EMI) shielding to prevent any electromagnetic signals generated during operation of the device from escaping and interfering with other electronic devices. Likewise, EMI shielding prevents any externally generated electromagnetic signals from entering the device and interfering with its operation. Such electromagnetic signals often develop as a field or as a transient within the radio frequency band of the electromagnetic spectrum, i.e., from between approximately 10 kilohertz and 10 gigahertz. Cellular or radio telephones in particular are required by law to adhere to electromagnetic compatibility (EMC) limits as laid down in Type Approval Specifications for Mobile Phones. xe2x80x9cEMCxe2x80x9d is defined as the ability of a device to function properly in its intended electromagnetic environment and not to be a source of electromagnetic pollution to that environment. To reduce or attenuate the effects of EMI, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device and to insulate the device or other target devices from other source devices.
It is known to use a cover or housing shell made of metal to absorb and shield any EMI radiation energy generated by the electronic device. One drawback of such metal covers is the additional weight and cost that is added to the electronic device. Additionally, the demand for increasingly smaller portable electronic handheld devices has caused manufacturers to integrate as many functions as possible, including EMI shielding, to reduce both manufacturing cost and the amount of raw material needed. One common method to provide EMI shielding in portable electronic devices is to mold or otherwise fabricate the covers or case of the device from metallized plastic; i.e., plastic having metal or other conductive fibers integrated into the plastic. Although such metallized plastic covers may provide adequate EMI shielding, they are less than satisfactory for making an electrical ground connection between the cover and an electrical ground circuit path on the electronic circuit board containing the operational electronic components of the electronic device. Special bonding techniques and additional labor steps are required to ensure electrical connection to the metal in the plastic material forming the cover. A further disadvantage of such metallized plastic covers utilized for EMI shielding is that it is very difficult, if not impossible, to separate the metal or other electrically conductive fibers from the plastic material into their respective base materials for recycling.
It is generally desired for the recycling of the electronic device covers that the cover be a homogeneous structure containing a single thermoplastic polymer blend free of contaminants such as metal fibers and the like, particularly if the product is to be used as raw material for production of a similar product application. Additionally, environmental legislation directed to the recycling of electronic devices may require reuse of materials as raw material for similar product applications, in which case it will be necessary to separate the materials of the cover into respective contaminant-free base materials.
Accordingly, it is an object of the present invention to provide an EMI shielded cover for portable electronic devices, such as cellular or radio telephones, wherein the EMI shield is releasable from the cover to facilitate separation of the EMI shield material from the cover material.
In one aspect of the invention, a cover for an electronic device has an interior chamber formed by intersecting wall surfaces and an electromagnetic interference (EMI) shield conforming substantially to the size and shape of the interior chamber is juxtaposed with the interior wall surfaces. The EMI shield is removable from the interior chamber for separating the cover and the EMI shield into their respective individual parts.
Preferably, the EMI shield is an electrically conductive fabric sheet.
Preferably, the electrically conductive fabric sheet is releasably fastened to the surface of the interior wall by an adhesive applied to a face of the fabric sheet.
Preferably, the adhesive are drops of glue in a spaced relation from one another on a face of the fabric sheet.
Preferably, the adhesive is an adhesive spray.
Preferably, the adhesive is heat activated.
Preferably, the electrically conductive fabric sheet comprises a woven mesh of conductive fibers.
Preferably, the conductive fibers comprise a silver coated thread.
Preferably, the electrically conductive fabric sheet is pliable and further is stretchable.
Preferably, the electrically conductive fabric sheet is of sufficient strength to resist tearing during removal from the cover.
Preferably, the cover is molded from a thermoplastic polymer blend.
Preferably, the EMI shield is fitted to the interior chamber of the cover subsequent to the molding of the cover.
Preferably, the EMI shield is pre-formed to the size and shape of the interior chamber.
Preferably, the EMI shield is a coating of a conductive polymer paint applied to the interior wall surface, which surface has modified whereby the adhesion force between the conductive polymer paint coating and the surface of the interior wall is less than the inherent cohesion forces of the conductive polymer paint coating itself to allow peeling of the conductive polymer paint coating from the interior wall surfaces.
Preferably, portions of the EMI shield are in electrical and mechanical contact with an electrical ground reference potential surface area on a substrate carrying electronic circuit components of the electronic device when the substrate is assembled with the cover.
In a further aspect of the invention, a method for providing an EMI shielded electronic device molded cover comprises the steps of providing an EMI shielding material that is conformable to the shape of the interior of the cover; locating the EMI shielding material in the interior of the cover; and releasably securing the EMI shielding material to the interior of the cover, whereby the EMI shielding material is removable to separate the EMI shielding material from the cover without substantial contamination of each by the other to facilitate recycling of the EMI shielding material and the cover into the respective raw material of each.
Preferably, the step of providing an EMI shielding material further comprises providing an electrically conductive fabric sheet having an adhesive applied to a face of the fabric sheet.
Preferably, the step of providing an EMI shielding material further comprises the steps of reducing the adhesion properties of the surface of the interior of the cover and providing a coating of a conductive polymer paint on the interior surface of the cover.